Too young to be trusted?

Recently this weekened, the case of two year old Caliegh Anne Harrison developed and gained coverege from several news stations. Harrison was playing on the beach on the day of her disappearance; her mother stepped away for a short time to retrieve a ball that had bounced away and Harrison was left with her four year old sibling, the only direct witness at the time of the girls dissapearance. Harrison's sister claims that the two year old girl was carried out to the ocean by the strong rip tide. Authorities are now struggling to determine if this story is likely or a fable manufactured by a child's imagination; which raises the question, at what age do we allow ourselves to believe the words of a child. Is there a reason to doubt the story of Harrison's sister? When it would be the most likely story and she was the only direct witness... Another issue within this case is the fact that Harrison's mother left two young children unattended near the ocean and that her story has been modified a couple of times since her original questioning. This story seems mysterious. I think that Harrison's four year old sister is reliable seeing as shes the only witness. In my experience, four year olds are very verbal, and though imaginitive, know when a situation is serious and know when to stop playing make believe.